Why Docker & what are its advantages over virtualization?

 Docker is actually an advance over virtualization without using its disadvantages. 

Virtualization uses some extra layers - hyper visor, Guest OS, it’s individual binaries and then the respective apps whereas using docker daemon replaces these layers and starts in milliseconds and achieves the same objective  

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So, in docker hosting an app becomes more easy as less resources and less time is required to launch whereas in case of Virtual machines, and entire hyper visor and guest OS layer is required to even launch a single app.